refactor(task): invoke Validate() in executor before submit

[bdchatham]

Summary

Adds e.params.Validate() to sidecarExecution[T].Execute() before the ToTaskRequest()/SubmitTask path. Catches malformed task params at executor-time as a structured terminal failure instead of as an opaque sidecar HTTP 400.

Closes the platform-engineer's deferred opportunity flagged in the cross-review of #192 ("Validate() is now defined on every wrapper but invoked nowhere in the executor path").

What it actually catches

For most task types Validate() returns nil (empty struct or trivial validation), so this is a no-op in practice. The two strict paths are:

• sidecar.AssembleAndUploadGenesisTask.Validate() — required-field + bech32 checks. Already invoked at planner-time at group.go:45, so this catch is defense-in-depth.
• configApplyTask.Validate() — delegates to seictl ConfigApplyTask.Validate(), which calls seiconfig.ValidateIntent. The planner doesn't currently call this; the executor now does.

Drive-by cleanups

• Drop a stale comment in sidecar.go about overriding Id "rather than on the wrapper's embedded TaskMeta" — TaskMeta no longer exists (removed in seictl#153, consumed in chore(deps): bump seictl to v0.0.46 #195). The comment was rot from refactor(task): drop taskParamser interface; use client.TaskBuilder directly #192.
• Fix TestExecuteGroupPlan_CompletesSuccessfully fixture — it built an AssembleAndUploadGenesisTask with only Nodes (missing required AccountBalance and Namespace). It passed before only because Validate() wasn't invoked; now we populate the required fields, which is the correct test intent for a happy-path submission test.

Verification

• GOWORK=off go build ./... clean
• GOWORK=off go test ./... — all packages pass

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